This application relates generally to laser systems and more particularly to a laser system capable of manipulation of its output.
Pulse compression, in particular, the ability to deliver pre-defined optical waveforms at the output of a laser or at a target location, is one of the cornerstones of ultrafast laser source development and the ever increasing number of applications that depend on ultra-short pulses. The shorter the pulses, the broader their bandwidth and the greater they are prone to Group Delay Dispersion (“GDD”). While air and most optical media introduce primarily linear group velocity delay, broad-band dielectric mirrors can introduce highly nonlinear GDD with spurious oscillations. Measuring and compensating for these distortions has been a challenge with traditional equipment and procedures. More recently, traditional autocorrelation measurements have been substituted by more advanced pulse characterization techniques such as Frequency Resolved Optical Gating (“FROG”) and Spectral Phase Interferometry for Direct Electric-Field Reconstruction (“SPIDER”). Advances in pulse shaping technology have led to the implementation of evolutionary algorithms for pulse compression, shaper-assisted versions of FROG, SPIDER and Spectrally and Temporally Resolved Up-conversion Technique (“STRUT”). It is believed, however, that STRUT has not been commercially accepted due to its inherent instability.
The paradigm of integrated pulse characterization and compression was realized when Multiphoton Intrapulse Interference Phase Scan, known as MIIPS®, procedures and equipment were commercially introduced. Various embodiments of MIIPS® are disclosed in U.S. Pat. No. 7,450,618 entitled “Laser System using Ultra-Short Laser Pulses,” issued on Nov. 11, 2008; U.S. Patent Publication No. 2009/0296744 entitled “Laser Based Identification of Molecular Characteristics,” which was published on Dec. 3, 2009; U.S. Pat. No. 7,609,731 entitled “Laser System using Ultra-Short Laser Pulses,” which was issued on Oct. 27, 2009; U.S. Patent Publication No. 2009/0238222 entitled “Laser System Employing Harmonic Generation”, published on Sep. 24, 2009; U.S. Patent Publication No. 2009/0207869 entitled “Laser Plasmonic System,” which was published on Aug. 20, 2009; and U.S. Pat. No. 7,567,596 entitled “Control System and Apparatus for use with Ultra-Fast Laser,” issued on Jul. 28, 2009; all of which were invented by Dantus et al., and are incorporated by reference herein. While MIIPS® is a significant improvement, in its most basic form sold in commercial production, it typically uses (but is not limited to) spectrometers measuring across an entire pulse spectrum in a frequency resolved, two-photon responsive manner, and relies on measurement and reconstruction of the spectral phase from its second derivative with respect to frequency.
Conventional sonogram measurements (such as frequency and time plots) of ultrashort laser pulses were disclosed by Fork et al., “Compression of Optical Pulses to Six Femtoseconds by Using Cubic Phase Compensation,” Opt. Lett. 12, 483-485 (1987), where amplified 50-fs pulses were cross-correlated with different spectral bands of a broadband continuum in order to characterize the compression of frequency-broadened optical pulses via a grating sequence. Furthermore, the idea of spectrally-resolved group delay measurements through cross-correlation with a reference pulse was disclosed in Chilla et al., “Direct Determination of the Amplitude and the Phase of Femtosecond Light-Pulses,” Opt. Lett. 16, 39-41 (1991). The detailed mathematical description followed in Chilla et al., “Analysis of a Method of Phase Measurement of Ultrashort Pulses in the Frequency-Domain,” IEEE J. Quantum Electron. 27, 1228-1235 (1991). A slightly modified version, where the resolving power was transferred onto the reference pulse and the entire up-converted spectrum was recorded, was disclosed in Foing et al., “Femtosecond Pulse Phase Measurement by Spectrally Resolved Up-Conversion—Application to Continuum Compression”, IEEE J. Quantum Electron. 28, 2285-2290 (1992). The eventually accepted name “STRUT” was introduced by Rhee et al. in “Chirped-Pulse Amplification of 85-Fs Pulses at 250 Khz with 3rd-Order Dispersion Compensation by Use of Holographic Transmission Gratings”, Optics Letters 19, 1550-1552 (1993). Many conventional constructions that rely on up-conversion in a nonlinear crystal are known as variants of STRUT. A somewhat distinct but very similar approach is to use two-photon absorption rather than up-conversion. The idea was disclosed in Albrecht et al., “Chirp Measurement of Large-Bandwidth Femtosecond Optical Pulses Using Two-Photon Absorption”, Optics Communications 84, 223-227 (1991).
One common disadvantage of these traditional approaches is the need for a separate split reference beam. It complicates the instrument setup and makes it difficult to characterize the pulse at the sample. Secondly, the compensation of measured phase distortions is delegated to different hardware components, for example, a simple prism-pair compressor, specially designed dielectric mirrors, or a pulse shaper, which add undesirable environmental and hardware variables into the analysis.
U.S. Pat. No. 6,327,068 entitled “Adaptive Pulse Compressor,” which issued to Silberberg et al., on Dec. 4, 2001, and is incorporated by reference herein, and “Femtosecond Pulse Shaping by an Evolutionary Algorithm with Feedback,” Applied Physics 63, 779-782 (December 1997), disclose the correction of a spectral phase using a genetic algorithm and measuring the maximum Second Harmonic Generation (“SHG”) signal, without pulse characterization. The disadvantage of this approach is that there are typically more than 100 pixels in a Spatial Light Modulator (“SLM”) pulse shaper and adjusting each one independently causes changes that are minimal with respect to the total SHG. Therefore, convergence toward the compressed pulse is time consuming and inaccurate. Other similar approaches that collect a non-linear optical signal from the entire pulse fail because changes in the central part of the pulse have a much greater weight than those in the wings of the pulse spectrum. Therefore, there is room for an improved, pulse characterization based accurate and efficient method for laser pulse characterization and compression that is less expensive because it does not require a spectrometer.